GRACE satellites during instrument integration
The two GRACE satellites are developed by Astrium GmbH and Space Systems/Loral (SSL). Astrium GmbH will build - under contract by JPL - major elements of the two flight satellites using heritage from the CHAMP mission. SSL provides the attitude control system, microwave instrument electronics and system and environmental testing.
Both satellites will be completely identical and shall differ only in the S-band radio frequencies used for communication with the ground and in the K-band frequencies used for the inter-satellite link. The satellite design features a simple and robust structure design, mainly based on Carbon Fiber Reinforced Plastic (CFRP) sandwich pannels with aluminium core and edge profiles for low thermal distortion. The outer shape of the satellite is optimized with respect to its aerodynamic behavior. This requires a symmetrical form with the center of pressure always in a single plane. Because the the spacecraft center of mass is on the same level as the center of pressure, the disturbances due to air drag and solar pressure are minimized.
All electrical units, the harness, the GN2 tanks, as well as the piping to the thrusters are located on both sides of the central equipment panel. The two main solar arrays are symmetrically canted to the equipment panel with two additional solar array panels on the satellite roof. The satellite body is closed at its front and aft side by sandwich panels of the same composite as the solar array panels. The aft pannel carries carries the occultation GPS antenna. while the front panel has the cut-out for the Ku/Ka-band horn. The nadir and zenith S-band antennae are mounted on brackets to minimize gain disturbance.
After successfull instrument integration, which has been finished in March 2000, both satellites will be shipped to IABG for environmental testing.
The following figures (courtesy by Astrium GmbH) describe the physical layout
of the GRACE satellites (click on figure to get better resolution):
View to the bottom of a GRACE satellite |
View to the top of a GRACE satellite |
The Attitude and Orbit Control (AOCS) System is composed of the necessary sensors, actuators, electronics, and software to provide adequate knowledge of spacecraft attitude during all phases of the mission, to generate on-board error signals to accurately maintain spacecraft attitude, and to provide necessary orbital control to satisfy the GRACE mission requirements. The AOCS is comprised of the following elements:
The instruments and flight systems are illustrated in the following block diagram (courtesy by Dick Stanton/JPL), where also the heritage from CHAMP and other small satellite missions is shown. For further details please refer to the description of the GRACE payload instrumentation.
